High-pressure discharge lamps (hereinafter referred to as “lamp”) have been used as a general light source for projectors. While a lamp is being lighted, a halogen cycle is caused wherein an electrode substance that is evaporated from electrodes of the lamp by the electric discharge between the electrodes is deposited on the electrodes again by the action of the halogen gas that is sealed in the light emission tube of the lamp. Reduction in the operating life of the lamp and the occurrence of flickering of the lamp can be minimized by appropriately controlling the halogen cycle. The halogen cycle can be controlled based on the lighting power of the lamp, the frequency at which the lamp is lit, and the temperature of the lamp, etc.
There are instances wherein the lamp cannot be lighted with rated power to cause a stable halogen cycle as when the lamp is used with low electric power to reduce the power consumption of the projector. In those instances, the lamp characteristics are degraded, i.e., the service life of the lamp is shortened and lamp flickering occurs. As a countermeasure, a method of recovering lamp characteristics by temporarily changing the lighting power of a lamp is disclosed in Patent document 1 (JP2009-093862A).
According the method disclosed in Patent document 1, if the lighting power of the lamp is lower than the rated power for a certain period of time or longer, then a lamp refreshing process is carried out to temporarily increase the lighting power of the lamp up to the rated power at appropriate time intervals. When the lamp refreshing process is carried out, the lamp characteristics can be recovered.
However, changing the lighting power of the lamp leads to changing the lamp luminance, resulting in a change in the luminance of images projected by the projector. Consequently, the user is required to determine the timing to start the lamp refreshing process and to recognize in advance that the luminance of projected images will change.
According to a projector wherein the amount of ight output from a lamp is adjusted by an optical diaphragm disposed in a light path, as disclosed in Patent document 2 (JP2004-264819A), the lighting power of the lamp and the optical diaphragm can be simultaneously controlled to adjust the luminance of projected images. The disclosed projector, however, is aimed at increasing the contrast of projected images, and serves to regulate the amount of lighting power and the amount of optical diaphragm relative to each other based on an input video signal, and is unable to deal with a change in the luminance of projected images in the lamp refreshing process.